PHYSIOGRAPHY OF BISHOP CONGLOMERATE 615 



While the features of the peneplain may be interpreted as due 

 to wind erosion, the writer is inclined to favor the hypothesis of 

 subaerial denudation under moist conditions. The reasons for 

 this preference are as follows: (i) Present conditions in the 

 region indicate only a moderate efficiency of wind erosion. The. 

 region at the present time is subjected to desert conditions, though 

 these are not now extreme. The winds are strong and blow almost 

 continually, especially during the day. Wind action is conspicuous 

 in places where conditions are particularly favorable, and a very 

 considerable amount of material is removed in this way, yet the 

 present features of topography are due rather to stream work than 

 to the wind. In places where the streams by cutting have exposed 

 softer rocks and these are of such a nature as to be easily eroded, 

 wind scour is noticeable, but otherwise not. Practically all the details 

 of topography are the result of water action. In southern Cali- 

 fornia where the average rainfall is less than five inches per year 

 the topography is almost entirely the result of stream action. 

 There is even less evidence of wind work than in Wyoming. All 

 the dissection of the mountains is of the type produced by streams 

 and there is little sign of wind erosion. It is true that the winds 

 do pick up and carry away considerable dust at certain times, par- 

 ticularly during dust storms, but their topographic effect as com- 

 pared with that of desert streams is almost negligible. For these 

 reasons the formation, by wind erosion, of a smooth peneplain like 

 that under discussion seems without parallel in present conditions. 

 (2) The conditions round the base of Aspen Mountain are thought 

 to indicate a moist climate at the time of planation. Those con- 

 ditions are indicated by the accompanying sketch, Fig. 7. At 

 the time of its development the peneplain extended with gentle 

 slope close up to the base of the mountain. Later, under condi- 

 tions of undoubted aridity, a mantle of piedmont gravels accumu- 

 lated round the base of the mountain. Since the arid climate 

 brought about the formation of the wash-apron it would seem that 

 at the time of planation the climate must have been moister, for 

 material resulting from weathering was then removed and the 

 peneplain developed close up to the hard rocks of the mountain. 



The basis of the assignment of the accumulation of piedmont 



